Nephilim are people touched by particularly powerful angels. The angel host can imbue the Nephilim with divine power but also control the Nephilim's thoughts and emotions. This is especially perilous to the controlled person when the angel is question is corrupted.
The wizard-priests of the lost kingdom of Ennon were said to be Nephilim, and thus they were able to sink the whole continent of Mercution into the sea a millenia before Heroica was founded. Heroica was also brought to the brink of destruction once when a dark angel called Lucifuge converted a group of heroes into Nephilim in an attempt to destroy the organization from within. This crisis became known as the Cruel Angel's Thesis.
A more benevolent case was of Periwinkle, a former prostitute turned a nun, who became a Nephilim for the Grey Angel Ophaniel in order to save her sisterhood from a vicious terror attack. In her case she temporarily gained visible angelic powers such as wings and robes made of pure light.